[llvm] [LoopVectorizer] Add support for partial reductions (PR #92418)

[Graham Hunter via llvm-commits]

================
@@ -331,6 +344,8 @@ class LoopVectorizationPlanner {
   /// Profitable vector factors.
   SmallVector<VectorizationFactor, 8> ProfitableVFs;
 
+  SmallVector<PartialReductionChain> PartialReductionChains;
----------------
huntergr-arm wrote:

I think this can be moved to the cost model, which is available to both the LoopVectorizationPlanner and the VPRecipeBuilder. It can be handled similarly to CallInsts, with a map between the relevant instruction (loop exit instr from the recurrence descriptor?) combined with VF, and a struct containing the relevant details. This also means you don't have to pass the Planner into the cost model.

https://github.com/llvm/llvm-project/pull/92418